Review article
Perianal fistulas: A review with emphasis on preoperative imaging

https://doi.org/10.1016/j.advms.2022.01.002Get rights and content

Abstract

Purpose

We aim to present a comprehensive literature review which focuses on the preoperative imaging of perianal fistulas.

Material/methods

Pelvic magnetic resonance imaging (MRI) and endoanal ultrasound (EAUS) are the two first-line imaging modalities for the preoperative evaluation of patients with perianal fistulas. We conducted a search in PubMed, Scopus and Google Scholar concerning articles comparing pelvic MRI with EAUS, which were published from 1994 until 2019.

Results

In most articles, pelvic MRI is superior to EAUS for the evaluation of perianal fistulas (especially for supralevator and extrasphincteric ones). Preoperative pelvic MRI is associated with statistically significant better results and prognosis after surgical treatment of the disease. Preoperative EAUS poses high sensitivity and specificity in identifying intersphincteric and transsphincteric perianal fistulas, as well as the internal opening of a fistula-in-ano. There is only one meta-analysis which compares the diagnostic accuracy of the two mentioned imaging modalities in preoperative fistula detection. Sensitivity of both - pelvic MRI and EAUS, is acceptably high (0.87). Specificity of pelvic MRI is 0.69 in comparison to EAUS (0.43), but both values are considered low.

Conclusions

Future well-designed prospective studies are needed to investigate the diagnostic accuracy of pelvic MRI and EAUS in the preoperative assessment of patients with perianal fistulas. Moreover, the combination of pelvic MRI and EAUS should also be studied, since several published articles suggest that it could lead to improved diagnostic accuracy. A novel treatment algorithm for perianal fistulas could arise from this study.

Introduction

A fistula-in-ano is an abnormal route of communication between a hole in the lumen of the anus or rectum (internal opening) and a hole in the skin of the perianal region (external opening). A perianal fistula is acquired, and its wall consists of fibrous connective tissue externally and granulomatous tissue intraluminally, with a variable degree of epithelization. The fistulous tract lies between the internal and external opening and may be linear or have a complex course with branches and secondary tracts [1,2].

Fistula-in-ano is a disease known and treated from antiquity. Hippocrates first described fistulotomy and seton placement in patients with perianal fistulas [3,4]. The incidence of the disease nowadays ranges from 1.7 to 2.3 cases per 10,000 persons/year in Europe, with a median age of 40 years. Male to female ratio is 2/1 [[5], [6], [7], [8], [9]]. Lifestyle and past medical history may be associated with an increased risk of the disease. Obesity (BMI>25 ​kg/m2), high daily salt intake, diabetes, hyperlipidemia, dermatosis, anorectal surgery, smoking, alcohol intake, sedentary lifestyle, excessive intake of spicy/high-fat food, very infrequent participation in sports and prolonged sitting on the toilet for defecation are recognized independent risk factors for perianal fistula formation [10,11].

Pathogenesis in most cases of fistula-in-ano is associated with cryptoglandular infection. In 40%–60% of patients treated for perianal abscess, the healing process leaves a perianal fistula and the disease evolves from acute to chronic phase [[9], [10], [11], [12], [13]]. Other, less common causes of perianal fistulas are anal injuries (traumatic – iatrogenic), Crohn's disease, malignancy of the anal canal or lower rectum, regional irradiation, leukemia or lymphoma, and specific infections (actinomycosis, tuberculosis, lymphogranuloma venereum, human immunodeficiency virus) [1,[9], [10], [11], [12], [13], [14]].

Park's classification is the prevailing system of anatomical classification for fistulas-in-ano [15]. There are 4 types of perianal fistulas according to their relationship to the external anal muscle: Type I or intersphincteric fistulas (45%), Type II or transsphincteric fistulas (30%), Type III or suprasphincteric fistulas (20%), and Type IV or extrasphincteric fistulas (5%) [[14], [15], [16]]. There is also a 5th type of fistula-in-ano described in the literature as superficial or submucosal [12,16,17].

In 1987, Fazio [18] classified perianal fistulas into simple and complex. Simple fistulas-in-ano include submucosal, intersphincteric and low transsphincteric (involving less than 30% of the anal sphincters, when they are not accompanied by secondary fistulous tracts and/or secondary abscess cavities). Complex fistulas-in-ano include high transsphincteric, supralevator, extrasphincteric, horseshoe and fistulas with secondary extensions or abscess cavities. Finally, complex perianal fistulas are considered to be those with high internal opening, non-healing, recurrent, anterior fistulas in women, and those associated with Crohn's disease, malignancy, or co-existing fecal incontinence [12,[16], [17], [18]]. The classification of perianal fistulas into simple and complex was also adopted by Standard Practice Task Force in 2005 [19].

A new classification system of fistulas-in-ano was presented at St James University Hospital in 2000 [20]. This system is based on axial- and coronal-level pelvic magnetic resonance imaging (MRI) findings and describes 5 types of perianal fistulas: Grade I - simple linear intersphincteric fistula; Grade II - intersphincteric fistula with abscess or secondary track; Grade III - transsphincteric fistula; Grade IV - transsphincteric fistula with abscess or secondary track within the ischiorectal fossa; Grade V - supralevator or translevator fistula. Patients with grade I and II fistulas are considered to have simple fistulas with favorable treatment results. Grade III and IV fistulas are considered complex with a potential threatening of continence after surgical treatment (Fig. 1). Grade V fistulas may arise from pelvic inflammatory disease [[20], [21], [22]] (Fig. 2).

The latest classification of perianal fistulas was introduced by Garg in 2017 [23], and it also is based on preoperative pelvic MRI and intraoperative findings: Grade I: low fistula - single tract (intersphincteric or transsphincteric); . Grade II: low fistula – multiple tracts or associated abscess or horseshoe tract (intersphincteric or transsphincteric); Grade III: high fistula – single tract (intersphincteric or transsphincteric) or anterior fistula in female, or associated morbidity (Crohn's disease, fecal incontinence); Grade IV: high fistula – multiple tracts or associated abscess or horseshoe tract (transsphincteric); Grade V: suprasphincteric or supralevator or extrasphincteric. Grade I - II perianal fistulas are considered simple and safe to be treated with fistulotomy. Grade III-V are categorized as complex fistulas and should be treated with a sphincter-sparing surgical procedure [[23], [24], [25]].

The accurate classification and thus knowledge of the topography of a perianal fistula and its relationship to the internal and external anal sphincter before surgery is critical for an effective treatment strategy. The use of diagnostic methods with high sensitivity and specificity in determining the anatomy of a perianal fistula preoperatively is essential [1,19,26,27].

Section snippets

Pelvic MRI

Pelvic MRI with intravenous use of gadolinium was first described in the late 80's and today it is the “gold standard” method for the preoperative imaging of all perianal fistulas, especially the complex ones [16,[28], [29], [30], [31], [32], [33], [34], [35]]. Advantages of MRI include high-contrast imaging of soft tissues, adaptable image field of view and its potential reconstruction in an axial and coronal plane. All these factors contribute to the diagnosis of the fistula-in-ano and the

Conclusions

Preoperative imaging evaluation of patients with perianal fistulas should include pelvic MRI and/or EAUS. Pelvic MRI is associated with statistically significant better results and prognosis after surgical treatment of the disease. Preoperative MRI has led to the discovery of new pathways of extension for perianal sepsis, a new type of complex perianal fistula (RIFIL) and more accurate classification systems for fistula-in-ano (St James classification, Garg classification). EAUS poses high

Financial disclosure

The authors have no funding to disclose.

The author contribution

  • Study Design: Nikolaos Varsamis, Christoforos Kosmidis, Grigorios Chatzimavroudis, Isaak Kesisoglou, Data Collection: Nikolaos Varsamis, Christoforos Kosmidis, Christoforos Efthymiadis, Fani Apostolidou Kiouti, Ioanna Papathanasiou

  • Statistical Analysis: Nikolaos Varsamis, Fani Apostolidou Kiouti

  • Data Interpretation: Nikolaos Varsamis, Christoforos Kosmidis, Grigorios Chatzimavroudis, Konstantinos Sapalidis, Christoforos Efthymiadis, Fani Apostolidou Kiouti, Ioanna Papathanasiou, Paul

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Aristotle University of Thessaloniki, Committee for Bioethics and Ethics approval for PhD research study, ID number: 2454, date: December 7, 2021.

The authors would like to thank Dr. Ioanna Papathanasiou from the Department of Biology, Medical Faculty, School of Health Sciences, University of Thessaly (Larissa, Greece) for providing useful insights that helped improve the study.

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